Search for Ultra-Relativistic Magnetic Monopoles with LHAASO-KM2A

Ultra-relativistic magnetic monopoles traversing the atmosphere are expected to initiate extensive air showers characterized by a muon deficit and a compact lateral profile. Using 1438.81 live days of data from the LHAASO-KM2A array, we present a search for these cosmic particles within the Lorentz factor range of ${10}^5$ to ${10}^7$. A machine-learning classifier utilizing lateral shower shape observables from the electromagnetic detectors is deployed to separate the signal from the hadronic background. No statistical excess above the background expectation is observed, and upper limits at the 90% confidence level are established on the isotropic monopole flux. The resulting upper limit is $2.6\times {10}^{-16}{\mathrm{c}\mathrm{m}}^{-2}{\mathrm{s}}^{-1}{\mathrm{s}\mathrm{r}}^{-1}$ at $\gamma ={10}^5$ and reaches $8.8\times {10}^{-19}{\mathrm{c}\mathrm{m}}^{-2}{\mathrm{s}}^{-1}{\mathrm{s}\mathrm{r}}^{-1}$ at $\gamma ={10}^7$, which improves upon the astrophysical Parker bound by over three orders of magnitude. This work establishes constraints on the monopole flux within a previously unconstrained velocity window.